The present invention relates to protective relay circuits for use in AC power distribution systems and more particularly to distance relays for transmission line protection.
Distance relays are well known in the art of protective relaying. For example, reference may be had to "The Art and Science of Protective Relaying", Mason, published by John Wiley & Son, Inc. (1956), particularly to chapter 14. Prior art distance relays include those which employ a phase comparator method of measurement. Exemplary circuits for phase comparator distance relays are disclosed in U.S. Pat. No. 4,034,269, issued to S. B. Wilkinson, the inventor of the present invention, on July 5, 1977; and U.S. Pat. No. 4,420,788, issued to S. B. Wilkinson and G. E. Alexander on Dec. 13, 1983. Both of these patents are assigned to the assignee of the present invention and are incorporated by reference into the present application as if fully set forth herein.
Positive sequence distance relays are utilized in protective systems for AC power transmission lines to detect three phase faults within a protected zone or portion of the transmission line. If such a fault is detected within the relay's zone or reach, the positive sequence distance relay will cause the operation of a three pole circuit breaker to trip all three phases of the transmission line. This is to be contrasted with a ground relay which defects faults between a phase and ground, again within a designated zone or reach; and, upon detection of such a fault, generates a signal which may operate a circuit breaker to trip the faulted phase only.
As indicated above, each particular positive sequence distance relay should only detect faults within its protected zone or reach. The parameters of a distance relay are commonly selected to correspond to the parameters of the transmission line. For example, the parameters are selected to provide a forward reach that may correspond to the forward distance of the zone of the transmission line under protection of the particular relay. It is desired that the distance relay operate within its selected forward reach thereby confining the protection of a particular distance relay to a selected zone within a protected system.
The desired operation of the distance relay may be hindered by the inability to distinguish between internal and external faults. Internal faults are those which occur within the protected zone or reach of the relay, whereas external faults are those that occur outside the zone or relay reach. For example, transients generated by capacitive voltage transformers or series capacitors in series compensated lines, often cause external faults to appear as internal faults to a distance relay.
In the past, many distance relays have employed phase angle comparator circuits. In some cases, these relays have employed amplitude comparators as well as phase angle comparators. In phase angle comparators, the characteristic is typically developed by measuring the angle between an operating signal, for example V.sub.OP =IZ-V, and a polarizing signal, V.sub.POL. This measurement may be accomplished by use of a coincidence logic circuit and a characteristic timer; see, for example, the above referenced U.S. Pat. No. 4,034,269. For the simplest phase angle comparator, the operating time is a function of the phase angle between V.sub.OP and V.sub.POL and the fault incident angle rather than the severity of the fault (i.e., the magnitude of the operating signal). Consequently, a fault just in front of the relay and a fault at a remote location (for example, 90% of the relay reach), would produce the same steady state timer input blocks of approximately one half cycle even though the operating signal for the close-in fault is much larger than that for the remote fault.
In the case of a fault just beyond the reach of the relay, the operating and polarizing signals are 180% out of phase and therefore produce no input to the characteristic timer. In addition, the magnitude of the operating signal is very small; therefore any errors in the operating signal, such as those caused by capacitive coupled voltage transformer errors, may cause the operating signal to reverse thus producing an undesirable operation of the measuring unit. In order to prevent such an overreach condition in a zone 1 relay, the operate signal must be sufficiently filtered to remove erroneous signals or the reach must be reduced.
Amplitude comparators have typically been used in negative sequence distance units and restrained overcurrent units. In this design, the minimum operating time is also determined by the setting of the characteristic timer as it is in the phase angle comparator design. However, even though this function operates on a comparison of signal magnitude, the speed of operation is not directly related to the fault severity. In this regard, the amplitude comparator circuit is similar to the phase angle comparator implementation.
Accordingly, it is an object of the present invention to provide a distance relay for detecting three phase faults in a protected zone of AC power transmission line which relay has an operating time which is based upon fault severity.
It is another object of the present invention to provide a distance relay which operates more quickly for a close-in fault as compared to a remote end fault in a protected zone of an AC power transmission line.
It is yet another object of the present invention to provide a distance relay with improved discrimination between internal and external faults thereby increasing the reliability of relay operation and the security of the protection system.
It is a further object of the present invention to provide a distance relay having improved discrimination between internal and external faults on series capacitive compensated AC power transmission lines.
It is still another object of the present invention to provide a distance relay having improved discrimination between internal three phase faults and close-in internal single line to ground faults for improved phase selection in single pole tripping schemes.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.